1. Field of the Invention
This invention relates in general to a speaker unit particularly for use in a headphone set and comprises an open air headphone.
2. Description of the Prior Art
Generally, open air type headphones include an ear cup as a housing which is perforated and an ear pad which is formed of urethane or similar material which has air permeability so that sound which is generated and obtained from a diaphragm of the headphone unit will be radiated therethrough to the user. Such open air type headphones can be worn without tiring or causing excessive pressure against the ears.
In such open air type headphones, however, since the sound is radiated to the outside of the headphone housing, stronger sound pressure is required as compared to closed type of headphones. Therefore, the open air type headphone is constructed so that the amplitude of the diaphragm is larger than that in closed headphone types.
A result is that in the event there is a very high output level of an amplifier connected to the headphone which is applied either accidentally when a tone arm having a reproducing needle is dropped on to a recorded disc or under any conditions where an abnormal high level signal is fed to a voice coil of the speaker, the diaphragm will be very strongly vibrated and the bobbin which carries the voice coil and is attached to the diaphragm will have a large movement. At such times, the bobbin can be caught on the yoke or pole piece comprising the magnetic circuit of the speaker and cannot return to its predetermined rest position. This can result in rendering the headphone inoperative and unuseable. In the prior art, the compliance or linearity of the edge portion of the diaphragm is intentionally selected so that it is smaller than the remaining portion of the diaphragm so as to protect the diaphragm from protruding. However, a speaker constructed in this manner will have less dynamic range and bad sound characteristics which is undesirable.